Add gswe_all_house_systems() function

This is required for Astrognome to configure house systems in the charts.
See gergelypolonkai/astrognome#36

docs/reference/swe-glib/swe-glib-sections.txt

@@ -211,6 +211,7 @@ gswe_antiscion_data_get_type
 <FILE>gswe-house-system-info</FILE>
 GsweHouseSystemInfo
 gswe_find_house_system_info_by_id
+gswe_all_house_systems
 gswe_house_system_info_new
 gswe_house_system_info_ref
 gswe_house_system_info_unref

src/swe-glib.c

@@ -828,3 +828,17 @@ gswe_find_antiscion_axis_info_by_id(
     return ret;
 }
 
+/**
+ * gswe_all_house_systems:
+ *
+ * Returns: (transfer container): All #GsweHouseSystemInfo registered with
+ * SWE-GLib. This list should not be modified nor freed and is only valid until
+ * a new house system is registered.
+ *
+ * Since: 2.1.0
+ */
+GList *
+gswe_all_house_systems(void)
+{
+    return g_hash_table_get_values(gswe_house_system_info_table);
+}

src/swe-glib.h

@@ -67,4 +67,6 @@ GsweAntiscionAxisInfo *gswe_find_antiscion_axis_info_by_id(
         GsweAntiscionAxis antiscion_axis,
         GError **err);
 
+GList *gswe_all_house_systems(void);
+
 #endif /* __SWE_GLIB_H__ */